Abstract

Feldsparphyric basalts commonly occur as a distinctive 'marker horizon' 300 to 500 m below the felsic volcanic units of many Archean greenstone belts. Within the Knee Lake greenstone belt, northern Manitoba, glomeroporphyritic basalts, characterized by large irregularly-shaped aggregates of plagioclase phenocrysts, occupy this stratigraphic position within the two lower-most volcanic cycles. A major chemical and/or tectonic break does not occur within the volcanic cycles, but rather, the chemical affinities of the Knee Lake volcanic rocks changes gradually (at a silica content of approximately 55%) from tholeiitic to calc-alkaline with increasing stratigraphic height. The accumulation of plagioclase phenocrysts suggests extensive crystal fractionation of the Knee Lake magmas. The gradual transition in affinity of the Knee Lake lavas from tholeiitic to calc-alkaline can be explained by near-surface fractionation under constant fO2 conditions.

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